The invention relates to a process for controlling the pressure on the belt of a CVT with of the bevelled pulley type in which, after setting of the belt, the transmission ratio and the transmission torque is determined by the pressure of the primary and secondary pulleys on the belt.
Continuously variable automatic transmissions of the belt with bevelled pulley type, hereinafter called CVT (continuously variable transmission), comprise the following units: starting unit, forward/reverse drive unit, variator, intermediate shaft, differential, hydraulic and electronic control device. Such a design is known from ATZ Automobiltechnische Zeitschrift 96 (1994), page 380. The variator, in turn, comprises a first bevelled pulley, with a hydraulic adjusting unit, situated on the input side, a second bevelled pulley situated on the output side, also having a hydraulic adjusting unit, and a belt extending between the bevelled pulleys. Each bevelled pulley, in turn, comprises an axially stationary, bevelled pulley half and an axially movable, bevelled pulley half. The bevelled pulley situated on the input side is hereinafter designated as the primary pulley. The bevelled pulley situated on the output side is hereinafter designated the secondary pulley. The axial position of the axially movable, primary pulley half determines the contact radius of the belt and thus the ratio of the CVT. The axial position of the axially movable, secondary pulley half determines the contact pressure of secondary pulley on the belt and thus the torque transmission capacity. The primary and secondary pulleys are adjusted by the pressure level in the respective hydraulic adjusting unit. These pressures are determined by the electronic control device by means of an electromagnetic pressure regulator located in the hydraulic control device. In this connection, a process for determining the contact pressure of the secondary pulley on the belt has been disclosed in Antriebstechnik 32 (1993), No. 9, pages 57 to 60. In the process, a problem that arises is that the torque-transmission capacity of the variator, when torque is first applied for, is not ensured, for example, when the engine is off or the transmission is in neutral or park. The reason for this is the back of pressure in the primary pulley, because in the starting ratio, the primary pulley is on the mechanical stop, that is, the adjusting space is pressureless and the belt runs on its minimal moving radius. As consequence of this, slippage can occur which, in turn, can result in damage to the belt and to the bevelled pulleys.
Another problem arises during reverse shifts. Reverse shifts are changes from a forward driving operation to a reverse driving operation and vice versa. Due to the reversal in direction of rotation, the course of the belt changes so that the components come to bear on each other in a reverse direction. The driver feels this occurrence as an impact which impairs the comfort when switching.